Musical instrument prohibition by judge, old (5)
I believe the answer is:
banjo
'musical instrument' is the definition.
(banjo is a kind of musical instrument)
'prohibition by judge old' is the wordplay.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'by' is a charade indicator (letters next to each other).
'judge' becomes 'j'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ban'+'j'+'o'='BANJO'
(Other definitions for banjo that I've seen before include "String instrument" , "Relative of the guitar" , "Stringed instrument like guitar with round body" , "Long-necked instrument" , "Stringed instrument beloved of old minstrel shows" .)